El Valle Boinás – Carlés Operation — 2014 Technical Report

The processing plant employs crushing, grinding, gravity concentration, flotation, carbon-in-leach cyanidation, and detoxification to recover gold, silver, and copper from blended skarn and oxide ores.

Report context

This technical report, dated September 26, 2014, describes the El Valle Boinás – Carlés Operation (EVBC) processing facilities. The plant was originally designed to treat 79 tph but was upgraded through process modifications to treat 95 tph or 2,280 tpd of ore from open pit mines. A maximum of 70 tph of skarn ore can be processed, and more than 2,000 tpd of the softer oxide ore can be processed together with the skarn ore. The plant operates with a zero discharge system using both fresh and recycled water. Energy is supplied from the grid and is sufficient to maintain operations of all circuits.

Processing route

Crushing and pebble crushing

Ore extracted from EVBC is stockpiled and classified according to mineralogy, gold, copper, and deleterious element grades. When possible, a suitable ore blend is prepared to feed the mill with material containing 2 g/t Au to 5 g/t Au and less than 1% Cu. A front end loader collects ore from stockpiles and delivers it to a crusher feed bin equipped with a 600 mm aperture slotted grizzly screen. The bin has an apron feeder that feeds a primary single toggle jaw crusher set to 153 mm. Spillage and crushed ore less than 153 mm are transferred to a conveyor belt equipped with a load cell type belt weightometer and discharged into a 100 ton mill feed bin. A level indicator and controller in the mill feed bin controls the apron feeder feed rate to the jaw crusher. The jaw crusher capacity is 200 tph. The mill feed bin can overflow and be diverted to an emergency storage stockpile with 1,000 tonnes capacity.

The semi-autogenous grinding (SAG) mill is fitted with pebble ports in the discharge end grate. The SAG mill product passes through a trommel screen with 12 mm apertures, and the oversize is transferred to a cone crusher. Pebbles are crushed to less than 12 mm and returned to the SAG mill via a conveyor belt with a load cell type weightometer. A moving belt magnet removes tramp metal, and a metal detector opens a by-pass chute ahead of the cone crusher if metal is detected. If the pebble crushing circuit is off-line, pebbles are diverted to an alternate conveyor belt and stockpiled.

Grinding and classification

Dry crushed ore at P80 less than 153 mm is fed to the SAG mill (5.5 m internal diameter by 2.3 m effective grinding length) by a conveyor belt with a load cell type belt weightometer. Pulp density is controlled by adding water to the SAG mill inlet. Oversize material is removed by the trommel screen and sent to pebble crushing. The undersize flows to a common pump box with ball mill discharge. The combined mill discharge is pumped to a cluster of seven 250 mm diameter Weir Cavex hydrocyclones for classification. Cyclone overflow reports to flotation at 40% solids with a size of P80 less than 74 µm. Cyclone underflow is split, with approximately 100 tph reporting to gravity concentration and the balance to the ball mill feed. The ball mill has a 3.85 m internal diameter by 5.65 m effective grinding length. If flotation is not necessary when processing low grade copper oxide ore, the flow can be diverted directly to cyanidation.

Gravity concentration

The cyclone underflow split to the gravity circuit passes over a vibrating screen with 2 mm polyurethane panels. Screen oversize returns to the ball mill feed. Screen undersize is split between a bank of 12 rougher double spirals (12LM3/2) and a Knelson-30 centrifugal concentrator (K30). The K30 concentrate is discharged to a concentrate tank; K30 tails feed two additional banks of 12 rougher double spirals. Concentrate from all three rougher spiral banks feeds a Knelson-20 (K20) concentrator. K20 concentrate is mixed with K30 concentrate in the concentrate tank; K20 tails are pumped to four cleaning double spirals (4LM3/2). The concentrate tank feeds a Knelson-12 concentrator that produces a high grade gravity concentrate. K12 tails are delivered to a concentrate dewatering cyclone ahead of a Holman 2000 shaking table.

The cleaner spiral concentrate feeds a re-cleaning double spiral (1LM3/2) and a retreat double spiral (1LM3/2). Re-cleaning spiral concentrate feeds two of four Holman vibrating tables. Concentrates from these tables are pumped to the dewatering cyclone ahead of the Holman 2000 shaking table, which produces a gold concentrate. Retreat spiral concentrate and mixed material from the re-cleaning spiral and Holman tables are pumped to two re-cleaning spirals (2xLM3/1). These re-cleaning spirals feed the other two Holman vibrating tables, producing a low grade gravity concentrate. Tails from all circuits are combined and pumped to the main dewatering cyclone. Cyclone overflow is split to the mill discharge pump box, gravity tails pumps, K30 feeding pump, flotation tails, and flotation dilution. Cyclone underflow returns to the ball mill.

Intensive lixiviation (ILIX) for gravity concentrate

The ILIX plant consists of a mechanically stirred reactor, an electrolysis tank, and two electrolysis cells. The high grade concentrate from the gravity circuit is fed to the reactor with a mixture of fresh water, soda, and cyanide. Cyanide dissolves gold in the concentrates. The leach solution containing gold is separated from solids by decantation. Solids are pumped to the ball mill, and the solution is sent to electrowinning.

Flotation

Flotation consists of two rougher stages, two scavenger stages, and two cleaner circuits. The mill cyclone overflow flows to a mechanically stirred rougher conditioning tank where flotation reagents are added. Slurry is conditioned for 2.5 minutes and reduced from 40% to 25% solids. Reagents added include lime to achieve neutral pH if necessary, Danafloat 123 promoter, and Dowfroth 250 frothing agent. Conditioned slurry flows by gravity to rougher and scavenger cells. Flotation tails are pumped to the CIL feed thickener. Rougher and scavenger concentrates are directed to a cleaning stage and then a re-cleaner stage. Re-cleaner concentrate gravitates to the concentrate thickener.

Thickening and filtration

The re-cleaner copper concentrate flows to a six metre diameter concentrate thickener. Flocculant is added as necessary. Thickener overflow flows to a lamella clarifier, and clear overflow is used as process water. Underflow from the thickener and clarifier is pumped to a transfer tank and then to one of two pressure filter feed tanks. Concentrate is pumped to a recessed plate pressure filter. Filtered concentrate at approximately 10% moisture is discharged, loaded onto trucks, and transported to the port warehouse in Gijon for sale.

Carbon-in-leach (CIL)

The CIL feed thickener underflow is pumped to a vibrating screen with 600 µm polyurethane panels to scalp off plastics and fibre. Screen underflow gravitates to the leach circuit consisting of six agitated tanks: one cyanide leaching tank and five CIL tanks. Each tank is 600 m³ in volume. Air is introduced via a low pressure blower through the agitator shaft. Total retention time is 24 hours. Leach slurry at 40% to 45% w/w solids is prepared by adding lime to control pH between 10.5 and 11.0, with cyanide in solution. Norit extruded activated carbon is suspended in the slurry. Each CIL tank outflow has a vertical cylindrical carbon retention screen with 700 µm by 700 µm wire screen panels. Loaded carbon is transferred from the first CIL tank to a wet vibrating screen for washing before elution. Carbon is transferred between other CIL tanks using air lifts. Slurry tails from the last CIL tank flow to the detox section.

Detoxification

CIL tails flow to two tanks for cyanide destruction using the INCO SO₂/Air process: one tank of 528 m³ live volume and one of 355 m³ live volume. Residual cyanide is destroyed and heavy metals are precipitated. Detox tails flow to a final tails safety screen that recovers any activated carbon should a carbon retention screen fail. Screen underflow is pumped to the tailings storage facility.

Desorption and elution

Loaded carbon from the first CIL tank is transferred via a vertical centrifuge pump to a cleaning screen and to an elution column, with slurry returning to the first CIL tank. Elution follows the Anglo-American AARL process using a nine cubic metre cylindrical column with capacity for four tonnes of carbon. The desorption stages include: HCl (3%) washing for 20 minutes at room temperature and atmospheric pressure; water rinsing for 120 minutes at 90°C and atmospheric pressure; desorption with NaCN/NaOH (3%/3%) for 20 minutes at 110°C and 350 kPa; water elution for 180 minutes at 110°C and 350 kPa; and cold water elution for 20 minutes at 60 to 110°C and 250 to 350 kPa. The elution process is completed in less than eight hours. If carbon is high in copper content, washing with a cold cyanide solution can be performed before elution.

Carbon regeneration

Stripped carbon is transferred by water pressure injector to a self-draining chute in a horizontal rotary regeneration kiln with capacity of 250 kg/h. The kiln is heated by propane to between 650°C and 750°C to thermally reactivate the carbon. Reactivated carbon is screened to remove fines before being returned to the last CIL adsorption tank. Regeneration takes approximately 15 hours.

Electrowinning

Eluate from desorption and elution is stored in the electrolyte tank. There are four electrolyte cells equipped with rectifiers, each with nine stainless steel wool cathodes. Electrolyte solution circulates through the cells and contains less than 10 g/t Au. Gold and silver are electrodeposited on the cathodes and washed off as EW sludge. Electrowinning takes less than 16 hours.

Smelting

Gold deposited as cathodic sludge is removed weekly and calcined in a kiln at 750°C before direct smelting. After calcination, gold and silver residues are mixed with fluxes (silica, borax, and nitrate) and smelted in a 0.04 m³ smelting furnace capable of reaching 1,200°C. Refined gold is cast into molds as doré bars.

Tailings disposal

Plant tailings are pumped to the tailings storage facility located within the old El Valle open pit mine. The tailings impoundment is lined with polyethylene liners and has an adequate pumping system. The EVBC Operation is a zero discharge facility. Plant tailings are currently not permitted as backfill at the El Valle-Boinás operations due to levels of arsenic in the material.

Key reported parameters

Note: YTD 2014 consists of seven months (October 2013 to May 2014).

Technical qualifications

The report notes that the main issue in maintaining gold, silver and copper production has been lower head grades in the ore blend. The exclusion of Carlés ore from the ore blend by the end of 2014 may have a long term impact on overall metal recovery, requiring further investigation and metallurgical testing. The process description was largely extracted from a previous 2012 technical report. The installation of pebble crushing has resulted in a reduction in increased grinding capacity (as stated in the original text). Products reported include doré (70–90% Au, 10–20% Ag, 2–10% Cu), flotation copper concentrate (100–300 g Au, 1,200 g Ag, 25–30% Cu), and gravity concentrate (200–300 g Au, 200 g Ag, 20–30% Cu), with typical gold recovery contributions of 30–60% from flotation, 40–70% from gravity, and 5–10% from intensive lixiviation, totalling 90–95%.

Source: El Valle Boinás – Carlés Operation , 2014 Technical Report, Section 17 Recovery Methods.